Studies of central nervous system electrophysiology and eye movements have revealed that monozygotic cotwins are more alike than dizygotic cotwins than would be expected from an additive genetic model. There is information that personality is influenced by the prenatal environment of twins as measured by placental type or dermatoglyphics. These findings bring up the possibility that the environmental covariance of MZ twins is greater than the environmental covariance of DZ twins due to prenatal effects. This study is to test the hypothesis that CNS electrophysiology, eye movement, personality and cognitive function variability are related to the placental type of twins.